Systems for the detection of an object in the path of a powered sliding gate using electromagnetic radiation are conventional and are depicted in the prior art. These systems usually contain both a source and a detector, and the position of an object in the path of the gate is determined by the absence of a normally detected signal. Such systems typically use light radiation with the source emitting a beam of light which is detected by the detector. The presence of an object in the path of the sliding gate is indicated when the light beam is broken by the object coming between the source and detector. An example of such a photoelectric system in use with elevator or train doors is depicted in the U.S. Pat. No. 1,947,079 to Ellis. In this system, the beams are aimed horizontally across the doorway.
It is also known to use non-visible light electromagnetic radiation (such as infrared radiation), microwave radiation, and ultrasonic waves to determine the presence of an object in a particular area, especially by reflected signals. Representative patents for these types of devices include the following: U.S. Pat. No. 3,852,592 to Scoville et al (infrared radiation); U.S. Pat. No. 4,029,176 to Mills (ultrasonic waves); and U.S. Pat. No. 3,877,002 to Cheal et al (microwave radiation).
There are a number of disadvantages to each of the prior art systems when they are used to detect the presence of an object in the path of a sliding gate. A major problem concerns where to locate the source and the detector. One place to position the source and/or detector is on a part of the sliding gate itself but this creates a major problem because of the requirement that the detector must move with the gate. This movement requires that the electrical connections to the source and/or detector must be able to move as well and subjects the electronic circuits and the electrical connections between the gate and the frame to wear or breakage from the door vibrations and the continuous flexing. In addition, when used outside of an enclosed area, dirt, dust and water are stirred up by the movement of the gate, deposit on the various components of the system and eventually cause decreased performance and eventual malfunction of the system.
Another location disclosed in the prior art for the source and detector is where the radiation can travel parallel to the direction of movement of the sliding gate at a position just inside or outside of the gate. However, in order to monitor a large portion of the path of the gate, a multitude of sources and corresponding detectors are necessary. Even in such cases, unless the sources and detectors are very closely spaced and a large number used, it is still possible for the radiation beams to miss the people or objects sought to be detected. In addition, in those devices where the gate has to travel a long distance this type of system causes the gate to stop upon the detection of an object even when the object may not be in danger of being struck at that time or when the object is moving out of the path of the gate when sensed.
Another disadvantage of the prior art systems is their complexity. In order to detect the presence of an object in the path of a sliding door, the prior art systems use either a plurality of sources and detectors, or the source must sweep the plane in which the gate moves. Both types of systems are complex, costly, difficult to maintain, and subject to failure as a result.
Other systems have been used which detect the presence of an object adjacent to the plane of movement of the sliding gate. These systems assume that any object near enough to be detected may be near enough to be in the path of the sliding gate. This is, of course, only an assumption, and it may well be that an object which is just in range of the sensor is sufficiently clear of the path of the sliding gate so that the sliding gate need not be stopped. Such systems, therefore, also suffer from the disadvantage of having to stop the gate unnecessarily.
Therefore, it is believed that none of the known conventional systems provide a reliable and simple electromagnetic radiation detection system for detecting when an object is located in the path of a sliding gate. Furthermore, none of the conventional systems provide a source and detector which is usable outside of enclosed areas.